Ganymede, Solar System's Largest Moon, Gets 'On The Map'

Global image mosaic of Ganymede (right) and the new geologic map created from it (left)

USGS Astrogeology Science Center/Wheaton/ASU/NASA/JPL-Caltech

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On August 4, the much-anticipated "Wonders of the Solar System" documentary will premier on the Science Channel.
Presenter and physicist Prof. Brian Cox will show you the hidden mysteries of our interplanetary neighborhood, as well as breathtaking sights of the planets, moons and the sun. Cox will also examine some of Earth's extreme environments to see how life has adapted, perhaps helping us understand whether life can exist elsewhere in our solar system.
To provide a taster of what you can expect from "Wonders," Discovery News has gathered some facts, figures and the best images of our solar system to assemble a special Wide Angle supporting this groundbreaking documentary.
Let's begin, in the orbit of the solar system's biggest planet, Jupiter...

Credits: NASA/Science Channel

View Caption+#2: Moons of Jupiter: Io

Welcome to Jupiter, the solar system's biggest planet and host to more than 63 peculiar moons -- a steady contender for moons compared to satellite-strewn Saturn.
From sulfur-spewing furnaces to ice-encrusted oceans, these Jovian satellites are anything but normal! Browse through 10 of our favorites here.
If you're looking for a fiery, volcanic spectacle, head to Io, the innermost of Jupiter's "big four" moons.
Jupiter's gravity pulls at Io so strongly that the land has tides of up to 300 feet (100 meters). This gravitational tug-of-war produces scorching heat and raises more volcanic activity here than anywhere else in the solar system.
Io has a sulfurous surface, and its volcanoes spew silicate magma, causing the hellish moon's surface look like a pizza.

Credit: NASA/JPL-Caltech/Galileo

View Caption+#3: Moons of Jupiter: Metis

While it doesn't look like much in this image (the highest-resolution available!), Metis is Jupiter's closest companion -- for now.
It's orbiting the gas giant at a distance of just 75,500 miles (128,000 kilometers) and moving faster than Jupiter spins. Metis is so close to Jupiter's surface, in fact, that it will gradually succumb to the planet's gravity and plunge into its churning clouds.
This same principle applies to man-made satellites orbiting the Earth; if their orbit is too low, they'll eventually fall.

Credit: NASA/JPL-Caltech/Galileo

View Caption+#4: Moons of Jupiter: Adrastea

Like Metis, Adrastea is on its way down, eventually: Its orbit is just 1,000 kilometers (621 miles) outside of its fellow doomed moon.
The two bodies probably provide lots of the material making up Jupiter's main ring, shown in this Galileo spacecraft image.
Adrastea is also tiny at a wee 12 miles (20 kilometers) in diameter.

Credit: NASA/JPL-Caltech/Galileo

View Caption+#5: Moons of Jupiter: Ganymede

If Ganymede orbited the sun instead of Jupiter, it would be a planet of its own -- it's even bigger than Mercury.
Its interior is made from layers surrounding a rocky core, and its surface is covered in water ice.
Although Ganymede doesn't have much of an atmosphere, it does have some ozone gas near its surface. This gas comes from charged particles in Jupiter's magnetic field smacking into the moon's icy crust.

Credit: NASA/JPL-Caltech/Galileo

View Caption+#6: Moons of Jupiter: Themisto

We're not entirely sure what Themisto looks like, but scientists know it one odd little satellite.
Unlike most other moons, Themisto is oblong and doesn't fit into the traditional groups of Jovian moons.
Because this body is so small -- just 5 miles (8 kilometers) in diameter -- astronomers in 2000 confused it for a new object when it was originally found in 1975.
Shown here is a speculative illustration of how the irregular moon might be shaped.

Credit: Courtesy of Damien Perrotin

View Caption+#7: Moons of Jupiter: Callisto

Along with Io, Ganymede, and Europa (which is yet to come in this slide show), Callisto is one of the four Galilean satellites.
These are the moons of Jupiter that Galileo discovered while looking through a telescope in 1610.
Callisto is about the same size as Mercury, and it's a heavily cratered moon that has almost no geological activity.
Callisto's surface may also be one of the oldest landscapes in the solar system, including Earth's moon -- about 4 billion years old.

Credit: NASA/Voyager

View Caption+#8: Moons of Jupiter: Thebe

Along with Metis and Adrastea, Thebe was discovered by scientists studying images from the Voyager spacecraft in 1979 and 1980.
It's closer to Jupiter than Io is, but it's not so close that it's in danger of losing its altitude.
The material in Jupiter's Gossamer ring probably comes from Thebe and another moon, Amalthea.

Credit: NASA/JPL-Caltech/Galileo

View Caption+#9: Moons of Jupiter: Amalthea

Speaking of Amalthea, shown here is an artist's rendition of the mysterious moon (inset: our best real view so far). Astronomers don't know much about it, but they do know it's the reddest body in the solar system.
Unlike the other three moons that lie within Io's orbit -- Thebe, Metis and Adrastea -- Amalthea wasn't discovered by the Voyager science team. Edward Emerson Barnard discovered Amalthea it in 1892, and it had been almost 300 years since anyone had discovered a new moon orbiting Jupiter.
Before that, the last person to discover a Jovian moon was Galileo.

Credit: NASA/Michael Carroll

View Caption+#10: Moons of Jupiter: S/2000 J11

This not-to-be-named moon marks an interesting dividing line in the wealth of satellites orbiting Jupiter.
S/2000 J11, named after it was found in 2000, moves in the same direction that Jupiter spins -- as do all the moons that are closer to the planet. Almost every distant moon, however, orbits in the opposite (retrograde) direction, as this diagram shows.
The only known exception is Carpo, sometimes known as Karpo, which was discovered in 2003.

Credit: University of Hawaii/Scott Sheppard/D

View Caption+#11: Moons of Jupiter: Europa

About two-thirds of the Earth is covered in water, but Europa has about twice as much water as our home planet.
Europa's might surface is covered in ice, but astronomers are almost convinced there is an ocean of liquid water underneath -- an environment shielded from Jupiter's intense radiation that might be hospitable to life.
A cold, salty planet might not seem like a good place to live, but there are species on Earth that thrive in just those conditions.
Slideshow originally posted Feb. 2009.

Credit: NASA/JPL-Caltech/Galileo

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When it comes to the multitude of moons that exist in our solar system, we often hear a lot about the few big stars of today’s scientific stage: Titan, Europa, Enceladus, Pluto’s most recently-discovered companions Kerberos and Styx, Mars’ moon Phobos… and of course our very own lovely moon, The Moon. But in the vast pantheon of heavenly satellites there’s one that looms above all the rest: Ganymede, the seventh moon of Jupiter and the largest moon in the entire solar system.

But just because Ganymede doesn’t make the headlines as often as its smaller cousins doesn’t mean it lacks fans in the scientific community, as evidenced by a brand-new geologic map released today by the USGS.

The comprehensive (and very colorful) map is the result of a project led by Geoffrey Collins of Wheaton College, and uses the most detailed images obtained of Ganymede by NASA’s Voyager 1 and 2 and Galileo spacecraft, executing imaging flybys of Jupiter and its moons in 1979, and the late 90s and early 2000s, respectively.

Using colors to differentiate the incredibly varied terrain on Ganymede, the illustrative map provides planetary scientists with the first solid evidence for distinct periods in the massive moon’s history: ancient, heavy cratering; tectonic upheaval; and then more recent settling and geologic decline.

“This map illustrates the incredible variety of geological features on Ganymede and helps to make order from the apparent chaos of its complex surface,” said Robert Pappalardo of JPL. “This map is helping planetary scientists to decipher the evolution of this icy world and will aid in upcoming spacecraft observations.”

ESA’s upcoming Jupiter Icy Moons Explorer (JUICE) mission is slated to launch in 2022 and orbit Ganymede around 2032, and this map will undoubtedly aid in mission planning and the targeting of specific objectives.

“The surface of Ganymede is more than half as large as all the land area on Earth, so there is a wide diversity of locations to choose from. Ganymede also shows features that are ancient alongside much more recently formed features, adding historical diversity in addition to geographic diversity.”– Geoffrey Collins, Geology Professor at Wheaton College, Norton, Mass.

The largest of Jupiter’s 63 named satellites, Ganymede has twice the mass of our moon and is even larger than the planet Mercury. In fact, at 3,280 miles wide Ganymede is bigger than Mercury and almost as large as Mars! Its complex icy surface is crisscrossed by dark regions covered with craters and lighter areas lined with grooves and ridges.

Ganymede even has a very thin oxygen atmosphere as well as its very own magnetosphere, generated by a molten core made up of heavy conductive metals. It’s been said that if Ganymede were to be orbiting the sun instead of Jupiter, it would easily be classified as a planet in its own right.

Discovered by Galileo in 1610, Ganymede is visible from Earth as one of the four Galilean moons seen alongside Jupiter (with Callisto, Io and Europa). With Jupiter so bright in the sky right now it’s fairly easy to spot Ganymede for yourself. All that’s needed is a small telescope or decent pair of binoculars (preferably mounted on a tripod) and a clear night sky… and a way to know what you’re seeing.